Many integrated audio amplifiers use a single supply (at supply voltage Vdd, typically 5V) and generate an output signal with quiescent voltage of about Vdd/2. Thus a.c. coupling is needed to provide an output signal referenced to ground, as required for connection to other equipment or speakers or headphones connected to ground. For low impedance loads such as speakers or headphones, these a.c. coupling capacitors need to be big to maintain good bass response (for driving a 16 ohm load, a 470 uF coupling capacitor is required for a −3 dB point at 20 Hz). The physical size of these coupling capacitors is especially significant for portable systems, and their cost is significant for low-cost systems. Also, the coupling capacitors need charging up (to Vdd/2) on power-up, and need to discharge on power-down or even during temporary power-saving idle modes of operation. This results in annoying clicks and thumps from the speakers or headphones unless special precautions are taken to prevent them. This generally requires extra switch transistors and resistors off-chip (since they need to connect to the terminal of the capacitor remote from the chip). These effects are observed, and extra components are needed to eliminate them, even if the output load is of higher impedance and the coupling capacitors are smaller, e.g. for line outputs driving 10 kohm loads.
Accordingly, it would be desirable to use an amplifier providing a drive out signal balanced at ground potential (0V), to avoid the space requirements and cost of the extra components. In other words, it would be desirable to use an amplifier whose quiescent output voltage was at ground potential, so that coupling capacitors were not required when driving a grounded load.
One attempted solution to this problem is disclosed in US2003/0138112A1. That document discloses a headphone driver system which incorporates a DC voltage-to-voltage converter arranged to generate a negative supply voltage from a positive supply voltage. The system incorporates headphone driver amplifiers, supplied with the positive supply voltage and the generated negative supply voltage, and arranged to produce output signals biased at ground potential (0V). Thus, although the system operates from a single voltage supply, large coupling capacitors between the amplifiers and the headphone inputs are not required. In a described embodiment, the voltage-to-voltage converter comprises a charge pump and relatively small (i.e. 1–10 uF) external capacitors. However, although the use of large coupling capacitors is avoided, the incorporation of the voltage-to-voltage converter clearly increases the complexity, and hence cost, of the amplifier system.
To reduce system cost, it would be desirable to use a low-cost negative supply generator, for example in the form of a simple, unregulated capacitor charge pump. For certain applications, it would be desirable to incorporate at least part of the negative supply generating circuitry on the same chip as other amplifier components. However, a problem with low-cost, simple negative supply generators is that they tend to be noisy (good output voltage regulation, e.g. using a linear post-regulator, which generally requires large decoupling capacitors of its own, would necessitate increased complexity and cost). Most regulators, especially low-dropout ones, have bandwidths below audio frequency, so it is difficult to suppress audio-frequency components of supply ripple or provide good load regulation at audio frequency without using large decoupling capacitors on input and output. Operation of a headphone amplifier, for example, from a clean positive supply and a noisy negative supply could lead to unacceptably high levels of noise on the generated output signal unless special precautions are taken to give the amplifier high supply rejection at audio frequencies and above.
Whether the negative supply is generated from the positive supply or is provided by an independent source, the integration of an amplifier operating from the dual (i.e. positive and negative) supply with other circuits on a single chip poses further problems. The circuits (for processing and handling of digital and analog signals) that one would like to integrate with the amplifier are typically designed to operate between a single positive supply and ground. Clearly, it is undesirable to have to modify established circuit arrangements for compatibility, i.e. for dual-supply operation.
Increasingly, and especially as a result of the growth of digital audio systems, it is becoming desirable to embed amplifiers on the same silicon substrate as other digital and audio signal processing circuits. Such integrated circuits are most cost-effectively implemented using smaller geometry processes than have typically been used in the past. It is desirable to implement the circuits using technologies which provide feature dimensions typically as small as 0.35 um, 0.25 um or even smaller. In such technologies, supply voltages are typically limited to 3.3V or 2.5V. Complementary MOS (CMOS), which utilises PMOS and NMOS technologies combined on a single silicon wafer, is perhaps the most cost-effective of the current technologies, compared say with BiCMOS or any specialised process incorporating additional device structures for high-voltage outputs. It would be desirable to provide an amplifier integrated with other circuits on a CMOS chip, or at least fabricated on a CMOS process with a small number of additional manufacturing steps. However, CMOS chips are conventionally arranged for single supply operation. The p-type substrate is connected to ground, so no n-type region (e.g. NMOS source or drain) can be biased more than a diode drop below ground.
Furthermore, the supply voltage limitations for CMOS devices conflict with legacy standards for analogue audio signal interfaces. For example, audio “line level” signals are conventionally required to be 2V rms (approximately 5.6V p—p, which is greater than even 3.3V). These requirements are unlikely to reduce, because of pressure for ever increasing performance and the need for headroom above fixed or increasing extraneous interference.
Thus, there is a need for audio output stages to be integrated with existing single-supply cell libraries to implement audio SoCs (which stands for “System-on-Chip” i.e. large chips with ˜1M gates implementing a DVD player or mobile phone, for example, on one or few chips), using standard-process CMOS at 0.25 um or so, with outputs swinging negative on an otherwise predominantly single-supply chip, with up to 2V rms o/ps depending on the application.
Embodiments of the present invention aim to provide amplifiers which overcome, at least partially, one or more of the above-mentioned problems associated with the prior art.